Django-adminfilters

* drop support for django < 2.1
* add support for django 3.1
* bug fixes

* add support for django 2.1
* updates filters templates

* add support for django 2.0
* new `ForeignKeyFieldFilter`
* drop support Django < 1.11

* add support for django 1.10
* fix empty variable in templates
* fixes :ghissue:`8` Fix filter id in templates

* django versions 1.4 to 1.9
* python 2.7, 3.4, 3.5
* broaden tests - test UnionFieldListFilter and IntersectionFieldListFilter (thanks PetrDlouhy)
* Demoproject made working and transformed to testing application (thanks PetrDlouhy)
* Various filters are included in demoproject (thanks PetrDlouhy)
* Coverage with coveralls.org (thanks PetrDlouhy)
* Readme update with actual filters (thanks PetrDlouhy)

* added MultipleSelectFieldListFilter, IntersectionFieldListFilter, UnionFieldListFilter